Sheets of aluminium alloy containing magnesium, suitable for producing bodies of cans by drawing and ironing, and method of obtaining said sheets

ABSTRACT

The invention relates to sheets of aluminium alloy containing magnesium, suitable for producing bodies of cans by drawing and ironing, and a method of obtaining said sheets. 
     The sheets are characterized in that they have, over 10 to 25% of their surface, uniformly distributed particles formed by amorphous aluminium oxides and crystalline magnesium and aluminium oxides, in the form of flat discs less than 5 microns thick and with a mean diameter distributed round a means value from 2 to 15 microns. 
     One of the methods of obtaining said sheets is characterized in that the strip is taken as it comes from the casting machine or after at least one rolling pass and subjected to chemical etching so as to reduce its thickness by a maximum of 2 microns, before being annealed in air at from 330° to 450° C. for at least 30 minutes. 
     The invention can be applied to the production of bodies of cans for fizzy or non-fizzy drinks, from metal sheets which do not stick in the drawing process, using a wide range of alloys.

The invention relates to sheets of aluminium alloys containingmagnesium, suitable for producing bodies of cans by drawing and ironing,and a method of obtaining said sheets.

A person skilled in the art knows that aluminium alloys can be castcontinuously, directly in strip form, in belt or cylinder type machinessuch as that described in French Pat. No. 1198006. The strips may be upto 2 meters wide and are generally from 5 to 30 mm thick. The advantageof this type of casting over casting in very thick plates is that itavoids a large number of rolling passes.

The strips thus obtained can be given smaller thicknesses, of down to afew microns, by a series of cold or hot rolling operations which may ormay not alternate with annealing operations. In particular, in caseswhere they are intended for the manufacture of bodies of cans, used e.g.for packing fizzy or non-fizzy drinks, the strips are converted tosheets a few hundred microns thick, circular discs are cut out of thesheets and converted to a cup by drawing and the combined action of apunch and die; after a so-called ironing operation to make the wallsthinner the cup can be used as the body of a can.

The change in shape from a flat disc to the body of a can involvesexerting strong forces and, particularly in the drawing process,accompanied by a sticking, or "galling" phenomenon which can be definedas adhesion of a soft metal to a hard one. In the present case aluminiumparticles are removed from the surface of the sheet and adhere to thesurface of the drawing tool, which is made of tungsten carbide. Theparticles, which are very hard drawn and therefore hard, make cracksappear on the body of the can, which reduce its mechanical strength, mayeven make it break, and in any case spoil its appearance.

Several methods have been recommended for avoiding this sticking andgiving the bodies of cans a suitable attractive appearance.

Thus U.S. Pat. No. 3,930,895, using a type 3004 alloy as defined by theAluminium Association, i.e. made up of Si 0.3%, Fe 0.5%, Cu 0.25%, Mn 1to 1.5%, Mg 0.8 to 1.3%, Zn 0.25%, balance Al, attributes thedifficulties encountered to the fact that continuous casting betweencylinders leads to the formation of Al-Mn particles with dimensions lessthan 2 microns; these are too small to have a cleaning effect on thetools and hence contribute towards dirtying them. It quotes the factthat, in the treatment of such an alloy obtained in plates byconventional casting, the particles have dimensions from 15 to 20microns, so there is no dirtying. But since the inventor neverthelesswants to have the advantages of continuous casting between cylinders,particularly as far as uniformity of the microstructure is concerned, herecommends increasing the size of the particles by modifying thecomposition of alloy 3004, through bringing the manganese content withinthe 2 to 3% range.

U.S. Pat. No. 4,111,721 also finds this galling phenomenon on sheets of3004 and 3003 and agrees that the dimensions of the Al-Mn and Al-Mn-Feparticles have to be increased to eliminate it. However, it teaches thatthis should be done not by modifying the composition but by applyingheat treatment, preferaby at a temperature of 620° C., for a period offrom 16 to 24 hours; the treatment may be applied either to the strip ascast or to the strip when it has already undergone a first series ofrolling passes.

French Pat. No. 2 505 365 again proposes modifying the composition ofthe alloy used, not in respect of the manganese but in respect of thesilicon, which it brings into the 0.3-0.6% by weight range.

French Pat. No. 2 525 047 filed by applicants recommends quite adifferent solution, comprising mechanically cleaning the surface of thestrip and modifying it by heating in air.

Although such processes have greatly reduced the sticking phenomenon,they have nevertheless not eliminated it completely. Hence applicants,in their research on solving the problem, have found that sheets whichare subjected to drawing and ironing have a special surface condition,and that this condition can be obtained with aluminium alloys of verydifferent compositions provided they have a common point, namely thepresence of magnesium.

This special condition is characterised in that the sheets have, over10-25% of their surface, uniformly distributed particles, formed byamorphous aluminium oxides and crystalline magnesium and aluminiumoxides, in the form of flat discs less than 5 microns thick and with amean diameter widely distributed round a mean value of from 2-15microns.

Thus according to the invention suitability for drawing and ironing is,as taught by U.S. Pat. No. 3,930,895, linked with the presence ofparticles of relatively large dimensions, coming close to those observedin plates. However, the similarity stops there, for the particles hereare very different from the point of view of distribution, compositionand shape. In fact:

the particles are not distributed right through the sheet but only overthe surface, and furthermore only over a portion of the surface.

the particles are not made up of intermetallics of the Al-Mn or Al-Mn-Fetype but of magnesium and aluminium oxides which are both amorphous andcrystalline.

the particles have a very special shape: that of flat discs of athickness limited to 5 microns and a mean diameter widely distributedround a mean value from 2-15 microns.

The particles of the invention can be recognised particularly byexamining the metal sheet with an electron microscope operating bytransmitted light, where they appear in the form of large clusters of ablack colour, regularly dispersed in a lighter matrix; micro analysisshows them to be made up essentially of amorphous and crystallinealuminium and magnesium oxides.

For the particles to be obtained, the starting alloy must thereforecontain magnesium. However, a magnesium content as low as 0.1% by weightis perfectly adequate, while contents of up to 5% do not prevent theresult from being obtained. But it should be realised that the inventioncan be applied to alloys also containing up to 2% of manganese and/or1.5% of silicon, to which elements there can be added at least one ofthe following: Cu, Zn, Cr, Fe up to a respective content of 0.5%, 0.5%,0.5% and 0.7% by weight, also refining elements such as Ti and B up to0.1% by weight for each.

Thus a sheet of aluminium alloy defined as 3004 in the AluminiumAssociation's standards, i.e. containing by weight: Mg 0.8-1.3%; Mn1.0-1.5%; Si 0.3%; Fe 0.7%, Cu 0.25%, Zn 0.25% and with a surfaceaccording to the invention, has produced over 200,000 cans with the sametooling equipment, without any breakages or grooves being found.

Similarly a reference 5182 alloy, i.e. an alloy containing 4.0-5.0% Mg;Si 0.20%; Cu 0.15%; Zn 0.25%., Fe 0.35% by weight and of the structureclaimed also enabled large numbers of cans to be produced withouttrouble.

It should be noted that sheets of aluminium alloys charged withmagnesium, which applicants have adapted for drawing and ironing inaccordance with the invention, have the enormous advantage of improvedtensile strength over sheets made of manganese alloys; they make itpossible to envisage reductions in thickness and thus improve the costprice of the cans.

To illustrate the invention, 2 figures which are micrographs of thesurface of sheets with 300-fold magnification are included in thisapplication.

FIG. 1 corresponds to a sheet of alloy 3004 which did not have anyparticles and, when ironed, gave rise to breakages from the formation ofthe 1000th can body.

FIG. 2 corresponds to a sheet of the same alloy with particles accordingto the invention, which has enabled over 200,000 bodies of cans to beproduced without any grooves.

The invention also concerns a method of obtaining said sheets. It ischaracterised in that the strip is taken either straight from thecasting machine or after at least one rolling pass, and subjected tochemical etching so as to reduce its thickness by a maximum of 2microns, before being annealed in air at from 330° and 450° C. for atleast 30 minutes. The etching conditions are preferably chosen so as togive a reduction in thickness of from 0.8 to 1 micron.

It is indeed known to etch aluminium strips resulting from continuouscasting with caustic soda. German Pat. No. DE 2 418 642 can be quoted inthis connection. However, the essential purpose of this patent is toeliminate irregularities in the strip, in the form of segregations,pores and other similar faults which have very troublesome effects onmetal strips, particularly when the strips are later converted tosheets, belts or thin sheets. This involves relatively strong etching,since the patents state that the quantity of material removed is from 10to 100 g/m², which corresponds to a thickness of 3.7 to 37 microns. Thistype of etching has nothing to do with that in the invention and it isnot moreover designed to prevent sticking in the process of ironingbodies of cans.

Applicants have made the surprising discovery that slight etchingreveals hard zones on the surface of the strip which, after beingannealed in air at from 330° to 450° C. for at least 30 minutes, haveparticles as described above and hence have the effect of:

reducing the coefficient of friction by hardening the surface,

reducing adhesion between tool and alloy,

permanently cleaning the drawing ring, thus giving the metal a releasingor self-cleaning action.

Etching may be effected by any etching agent but preferably a solutioncontaining from 0.1 to 10 g/l of NaOH; the strip is submerged in it longenough not to remove more than 2 micron thickness of metal. The etchingtime depends on the composition of the alloy used and the rollingoperations undergone by the strip. Etching can be carried out at ambienttemperature, i.e. at around 20° C., although it can take place at anytemperature below 100° C. in order to shorten the treatment. The 70°-80°C. range is preferable.

Additional tests have shown that the results could be further improvedby brushing as in French Patent No. 2 525 047, after the etching of thestrip and before annealing. The brushing appears to have the effect ofimproving the distribution of hard zones in the whole surface of thestrip and consequently generating more homogeneous non-stick surfaces.

Optimum annealing conditions are obtained within the temperature rangefrom 350° to 400° C. and the time range from 1 to 2 hours.

The invention can be illustrated by the following example:

A strip 1100 mm wide, 8 mm thick and of the following composition byweight:

    ______________________________________                                        Si      Fe     Cu       Mn   Mg     Ti   Al                                   ______________________________________                                        0.28    0.42   0.10     1.00 1.05   0.02 Balance                              ______________________________________                                    

is obtained by casting between cylinders.

It undergoes the following conversion sequence:

homogenisation at 600° C. for 10 hours

cold rolling to 2.7 μm

annealing for 2 hours at 350° C.

cold rolling to 1.2 m

etching: 45 seconds in a 5 g/l solution of NaOH at 70° C. so as toremove a metal thickness of 0.8 microns

brushing

annealing for 2 hours at 400° C.

cold rolling to 0.33 mm.

The metal is then used to make bodies of cans 66 mm in diameter and witha mean height of 130 mm, by drawing and ironing.

In a series of 200,000 cans none have any grooves.

On the other hand, metal which has undergone the same conversionsequence without any etching does not produce cans without grooves;after the first 500 cans the grooves increase with the number of cansproduced, and after 1,000 the walls of the can bodies are found tobreak.

I claim:
 1. Sheets of aluminum alloy containing about 0.1 to 5% byweight magnesium suitable for producing bodies of cans for fizzy ornon-fizzy drinks by drawing and ironing, obtained by strip casting,followed by heat treatments and rolling passes, said sheets comprisingover 10 to 25% of their surface, uniformly distributed particles ofamorphous aluminum oxides and crystalline magnesium and aluminum oxides,in the form of flat discs less than 5 microns thick and having diametersdistributed around a mean value from 2 to 15 microns.
 2. The sheets ofclaim 1, containing at least one of the elements Mn up to 2% by weightand Si up to 1.5% by weight.
 3. The sheets of claim 2, containing atleast one of the elements Cu up to 0.5% by weight, Zn up to 0.5% byweight, Cr up to 0.5% by weight, and Fe up to 0.7% by weight.
 4. Thesheets of claims 1, 2 or 3 containing up to 0.1% of boron and 0.1% oftitanium by weight.
 5. A method of producing sheets of aluminum alloycontaining 0.1 to 5% by weight mangesium comprising taking strip of saidalloy as it comes from a casting machine or after at least one rollingpass, subjecting said strip to chemical etching so as to reduce itsthickness by a maximum of 2 microns, and annealing in air at from 330°to 450° C. for at least 30 minutes, to produce sheets of said alloyhaving, over 10 to 25% of their surface, uniformly distributed particlesof amorphous aluminum oxides and crystalline magnesium and aluminumoxides, in the form of flat discs less than 5 microns thick and havingdiameters distributed around a mean value of 2 to 15 microns.
 6. Themethod of claim 5, wherein the reduction in thickness is from 0.8 to 1micron.
 7. The method of claim 5, wherein etching is effected with asodium hydroxide solution.
 8. The method of claim 7, wherein the sodiumhydroxide solution is at a concentration of from 0.1 to 10 g/l.
 9. Themethod of claim 5, wherein etching is effected at a temperature from 20°to 100° C.
 10. The method of claim 9, wherein the temperature is from70° to 80° C.
 11. The method of claim 5, additionally comprisingbrushing between the said etching and said annealing.
 12. The method ofclaim 5, wherein annealing is carried out at from 350° to 400° C. 13.The method of claim 5, wherein the annealing period is from 1 to 2hours.